Brushless three-phase dc motor

ABSTRACT

A three-phase brushless dc motor includes a permanent-magnet rotor magnet arrangement having at least four poles and a Y-connected, or star-connected, three-phase stator winding. The winding&#39;s phases are arranged non-overlapping in slots of a slotted stator, the currents flowing in the three phases being controlled via at least three semiconductor elements by at least three magnetic-field-sensitive rotor position sensors. Each sensor is associated with a respective two of the winding&#39;s three phases and triggers a commutation which switches off the current in one of the associated two phases and switches on the current in the other of the associated two phases. The sensors are located to sense the permanent-magnet flux emanating from the rotor poles themselves. The rotor position sensors are provided at special angular locations on the stator. Each sensor is provided at an angular location at which there is not to be found, neither prior to nor subsequent to the commutation associated with that sensor, any energized stator pole; this may mean that the sensor location is (i) an angular location at which no energizable stator pole whatever is present, and it may mean that the sensor location is (ii) an angular location at which an energizable stator pole is in fact present, but during motor operation this energizable stator pole is in an unenergized state prior to a commutation associated with the particular sensor in question and likewise is in unenergized state subsequent to that commutation.

This application is a Continuation of application Ser. No. 08/154,383,filed Nov. 18, 1993, now U.S. Pat. No. 5,418,416, which is aContinuation of application Ser. No. 07/902,474, filed Jun. 19, 1992,now abandoned, which is a Continuation of application Ser. No.07/620,645, filed Nov. 30, 1990, now abandoned, which is a Continuationof application Ser. No. 07/066,471, filed Jun. 26, 1987, now abandoned,which is a Continuation of Ser. No. 06/607,688, filed May 7, 1984, nowabandoned.

BACKGROUND OF THE INVENTION

The invention concerns a three-phase brushless dc motor with apermanent-magnet rotor magnet arrangement having at least two pole pairsand a star-connected, three-phase stator winding, the winding's phasesbeing arranged non-overlapping in slots of a slotted stator, with theircurrents being controlled via at least three semiconductor elements byat least three magnetic-field-sensitive position sensors, the latter inturn being controlled by the rotor magnet arrangement.

With motors controlled in this manner, especially in the case of motorsoperating at high power, there arises the problem of the field from thestator winding influencing the magnetic-field-sensitive position sensorsalthough in theory, of course, these sensors should be responsive onlyto the rotor magnet poles. As a result of such influence, thecommutation time points are in undesirable fashion shifted from theirpredetermined optimal times of occurrence. This is because any part ofthe stator-winding field that happens to be incident upon one of therotor-position sensors is wrongly interpreted by the latter as part ofthe flux coming from a rotor-magnet pole. Accordingly, this problembecomes more severe, the higher the winding's current.

SUMMARY OF THE INVENTION

Therefore an object of the invention is to so design a dc motor of thestated type as to substantially prevent a shifting of the commutationtime points under the influence of the currents flowing in the statorwinding.

According to the invention this object is achieved in a surprisinglysimple manner by choosing the respective angular locations for theposition sensors in a special manner relative to the coils of the threephases. Usually, each position sensor triggers or in another mannercommands commutation or switchover of the winding's current from a firstto a second one of two phases associated with that particular positionsensor. In accordance with the present invention, in such event, eachsensor element is placed at an angular location on the stator at whichthere is not to be found, neither prior to nor subsequent to thecommutation associated with that sensor, any energized stator pole. Thismay mean that such sensor's location is (i) an angular location that iscircumferentially intermediate two circumferentially spaced, neighboringenergizable stator poles, to thereby be an angular location at which anenergizable stator pole is not present; and the foregoing may mean thatthe sensor's location is (ii) an angular location at which anenergizable stator pole is indeed present, but during motor operationthis stator pole is in an unenergized state prior to a commutationassociated with the sensor in question and likewise is in unenergizedstate subsequent to that commutation.

With the motor according to the invention the magnetic-field-sensitiveposition sensors remain uninfluenced by the stator winding's fieldduring the commutation. Even in the case of higher winding currentsthere does not occur an undesired displacement of the commutation timepoints.

In accordance with a further feature of the invention the number ofstator poles stands in the ratio 3:4 to the number of rotor poles, eachof the stator poles having a breadth of substantially 180°-el. As aresult a chording is avoided. A particularly high efficiency isachieved. The torque developed by the motor is substantially uniform.

It has proved especially advantageous to locate each position sensorsubstantially midway, considered in the circumferential direction,between those neighboring coils as between which the commutation of thewinding's current occurs under the influence of that particular positionsensor.

According to a further embodiment each position sensor can also belocated substantially on the radial symmetry axis of a stator polecarrying a coil of the one phase that is not involved in the commutationoperation triggered by that particular position sensor.

The motor can be designed as a three-pulse motor or else as a six-pulsemotor, in the latter case it having preferably at least four magneticpole pairs.

Preferably, at the air gap, the space remaining in the circumferentialdirection between each two neighboring, preferably 180°-el.-wide statorpoles is substantially filled up by an unwound auxiliary stator pole.The auxiliary stator poles very effectively avoid a magnetic jolt,because an approximately uniform induced voltage is obtained over arelatively large angle, which means a uniform torque at constantcurrent. Without such auxiliary poles between 180°-el.-wide statorpoles, in the case of a ratio of stator poles to rotor poles of 3:4 thestator poles would be, functionally, wider than 180°-el. because a greatpart of the rotor's magnetic field appearing in the pole gaps, would beattracted to the stator poles. There would set in an undesired manner achording action.

In the case of the motors set forth above--but also in general in thecase of brushless dc motors with a permanent-magnet rotor magnetarrangement and a slotted stator carrying a stator winding wound withoutoverlap, the stator having a succession of poles that are of one piecewith one another, e.g. stamped out in conventional manner from Dynamosheet metal--it is desirable that, on the one hand, the slot openings bekept small but, on the other hand, that the slot openings be largeenough to facilitate insertion of the winding's constituent coils duringthe fabrication. In accordance with the invention this problem is solvedin that the auxiliary poles are provided as separate parts which can beinserted between main poles and be connected to the stator afterwards.During the winding procedure the auxiliary poles are left off. As aresult the winding can be installed in the stator slotsunproblematically. The auxiliary poles are installed only when thestator windings have been formed. These later installable auxiliarypoles can advantageously form the aforesaid unwound auxiliary poles.

In accordance with a further feature of the invention the auxiliarypoles are insertable into recesses of the stator winding's core. Whereasthe latter advantageously involves, in conventional manner, a stack ofsheet metal, each auxiliary pole is preferably formed as a one-piecepole body. In particular the auxiliary poles can be formed from solidmaterial or as sintered bodies. Because the auxiliary poles, incorrespondence to their relatively short circumferential extent, acceptonly a relatively small magnetic flux, eddy-current losses, inparticular, remain low even in the case of solid auxiliary poles.Manufacture from sintered iron has the advantage that by means ofpowder-metallurgy techniques very exact shapes can be manufacturedwithout there being a need to thereafter do further material-removingmachining work. Furthermore, for electro-technical applications thereare commercially available also suitable siliconized iron types, such ase.g. from the Vakuumschmelze Company under the trade name "Trafoperm".

The auxiliary poles can advantageously be provided with recesses suitedfor accommodating the position sensor; the latter can involve, inparticular, Hall generators, Hall-IC's or similar magnetic sensors. Inthe case that sintering techniques are used such recesses can be formedin particularly simple manner.

The invention is explained in greater detail below with respect topreferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts, schematically, a sectional view of a dc motor accordingto the invention,

FIG. 2 is a view similar to FIG. 1 for a modified embodiment of theinvention,

FIG. 3 is a sectional view through an auxiliary pole corresponding tothe line III--III of FIG. 2, and

FIG. 4 depicts a set of waveforms referred to in describing thepreferred mode of operation of the motor.

DESCRIPTION

The three-phase brushless dc motor of FIG. 1 has a rotor 10, rotatablymounted in a manner not illustrated in detail, with a permanent-magnetrotor magnet arrangement 11. The rotor magnet arrangement 11 ispreferably formed from a rubber-magnet strip, i.e. a one-piece stripmade of a mixture of hard ferrite, e.g., barium ferrite, and elasticmaterial. The magnet strip is radially magnetized; its magnetizationpattern has the shape of a trapezoid, or approximately a trapezoid,extending over the pole pitch, with relatively small pole gaps. It formsin the illustrated embodiment four pole pairs which at their outerperipheral surfaces constitute, alternately, magnetic north poles 12 andmagnetic south poles 13. It is to be understood that other magneticmaterials can be used too, and that the rotor magnet arrangement canalso be assembled from individual magnetic plates.

The rotor 10 is encircled by a stator 15, preferably in the form of alaminated stack of sheet metal, forming a cylindrical air gap 16. Onlyone-half of the stator 15 is depicted; the other half is configuredsymmetrically in correspondence thereto. The stator 15 has six T-shapedmain poles 17. The pole faces 18 of the main poles 17 face the air gap16 and each extends for an angle of 180°-el., i.e., the width of each ofthe six main poles is, at the air gap 16, equal to the width of each ofthe eight rotor poles 12, 13. In this way, at the air gap 16, thereresult between the main poles 17 gaps which are each 60°-el. wide. Thesegaps are substantially filled up by auxiliary poles 19, i.e., the polefaces 20 of the auxiliary poles 19 extend from a breadth of almost60°-el.; they end at a small distance, in the circumferential direction,from the respective neighboring main-pole face 18. Each of the mainpoles 17 carries a stator coil, of which in FIG. 1 the three statorcoils 22, 23, 24 are depicted. Corresponding stator coils--which can beconnected in series with their respective diametrically opposite statorcoils 22 or 23 or 24--are provided on the three non-illustrated mainpoles. The stator coils altogether form a star-connected, three-phasestator winding whose constituent coils do not overlap one another. As aresult the axial ends of the coils are specially short in the axialdirection, which is advantageous not only for spatial reasons but alsoleads to low winding resistance. In the arrangement of FIG. 1 the commonor star junction of the stator winding is connected via a line 25 to thepositive side +U_(B) of a voltage source; the star junction is connectedto a respective first end of each of coils 22, 23, 24. The other end ofeach of these coils is connected, via its associated series-connected,diametrically opposite coil (not shown) to a respective semiconductorswitch 26, 27 or 28, and then via a respective one of these switches tothe negative side -U_(B) of the voltage source. For the commutationoperations the semiconductor switches 26, 27, 28 are controlled by themagnetic-field-sensitive position sensors 30, 31, 32. The positionsensors can in particular be Hall generators or Hall IC's which are, inturn, controlled by the rotor magnet arrangement 11.

As a result of the given geometry, the position sensor 30 can bearranged on the stator at eight different angular locations along theair gap 16, of which four locations are denoted by 30a, 30b, 30c and 30din FIG. 1. The other four possible angular locations are locateddiametrically opposite to respective ones of the locations just stated.It has been found that one can in a simple way avoid the positionsensors being exposed to the field from the stator coils, and avoid theresulting undesired shifting of the commutation time points, by locatingthe position sensors 30, 31, 32 at the air gap 16 at speciallydetermined angular locations. As defined earlier in the generaldiscussion of the invention further above, each sensor is placed at anangular location at which there is not to be found, neither prior to norsubsequent to the commutation commanded by that sensor, any energizedstator pole; as stated earlier and as will become clearer below, thismay mean that such sensor's location is (i) an angular location at whichno energizable stator pole whatever is present; likewise, this may meanthat the sensor's location is (ii) an angular location at which anenergizable stator pole is in fact present, but during motor operationthis stator pole is in unenergized state prior to a commutationassociated with the sensor in question and likewise is in unenergizedstate subsequent to that commutation. The position sensor 30 triggers orcommands the commutation from the stator coil 22 to the stator coil 23,rendering the semiconductor switch 26 non-conductive and rendering thesemiconductor switch 27 conductive. The criterion stated just above isfulfilled for position sensor 30 if the latter is arranged at theangular positions 30a or 30c; in contrast, it is not fulfilled at thepositions 30b and 30d. The position 30a is located on the radialsymmetry axis of the main pole 17 carrying the coil 24, i.e., a coil ofthe one phase that is not involved in the commutation operationtriggered by position sensor 30. The second advantageous position 30cfor the position sensor 30 is at the auxiliary pole 19 located angularlyintermediate the two stator coils 22 and 23; these are coils of the twophases that are involved in the commutation or switchover commanded byposition sensor 30. In contrast, the two further positions 30b and 30dfor the position sensor 30 do not satisfy the aforesaid criterion. Atposition 30b the position sensor 30 would be exposed to the magneticfield from coil 23 after the commutation operation, whereas at position30d the position sensor 30 would be exposed to the field from coil 22prior to the commutation in question.

Corresponding remarks apply to the other two position sensors 31, 32;the positions in principle possible for these within the illustratedregion of 180°-mech. are denoted by 31a, 31b, 31c, 31d and 32a, 32b,32c, 32d, respectively. Here again, of these positions only thepositions 31a and 31c for sensor 31 and 32a, 25 and 32c for sensor 32,fulfill the criterion in question.

The foregoing discussion of FIG. 1 presupposes a certain known type ofthree-pulse operation, i.e. the rendering conductive at any given timeof only one of the winding's three phases, the flow of current througheach of the winding's phases always being in the same direction, asdepicted graphically in FIG. 4 and reviewed further below. The statorcoils 22, 23, 24, and their non-illustrated diametrically oppositepartners, and the semiconductor switches 26, 27, 28 accordingly form acircuit configuration which can be designated as half of a bridgecircuit. However, the invention is not limited thereto. The describedmotor can instead operate with a full bridge circuit permitting areversal of the direction of the current in each phase (such a fullbridge circuit is for example known from FIG. GB of DE-OS 30 44 027),and the motor can thusly be operated in six-pulse fashion, in which caseat any given time two of the winding's phases carry currentsimultaneously. Referring to FIG. 1 it may be noted that, in the case ofsix-pulse operation, only the positions 30c, 31c, 32c fulfill theinventive criterion.

The use of at least eight permanent-magnet poles has furthermore theadvantage that the forces exerted on the rotor shaft are symmetricalwith respect to the motor axis.

The Hall element positions as discussed above have particularsignificance when the motor of FIG. 1 is operated in the particular typeof three-pulse fashion shown in FIG. 4, although persons skilled in theart will understand that other and equivalent contacts may also beemployed. FIG. 4A shows the output voltage of one of the motor's threeHall elements. This voltage is cyclical and has a period equal to360°-electrical. The FIG. 4A voltage is applied to a comparator, orother conventional pulse shaper, to yield the better defined voltagewaveform of FIG. 4B, each pulse of which lasts for 180°-electrical. Thesame applies for the other two Hall elements, but preferably theirrespective pulse trains are phase-shifted one from the other by120°-electrical, as shown for the three Hall elements in FIG. 4C, i.e.,due to the locations of these three elements. The set of three pulsetrains of FIG. 4C is applied to a logic circuit to generate threedifferent pulse trains shown in FIG. 4D. The pulses in each of thesetrains have a duration of 120°-el., a period of 360°-el., and the threepulse trains are phase-shifted one from the next by 120°-el. Each of thethree pulse trains is used to render conductive a respective one of thethree transistors 26, 27, 28, so that FIG.4D also represents therespective conduction times of these transistors. FIGS. 4E1, 4E2, 4E3depict the theoretically possible and (in the shaded areas) the actualtorque contributions of respective ones of the three coil systems, thetype of three-pulse operation shown in FIG. 4 being the type thatutilizes less than the full torque theoretically possible. Namely, ifthe respective transistor of one of these phases were always conductive,the associated torque contribution would be sometimes in the correctrotation direction (shown as positive) and sometimes in the non-desiredrotation direction (shown as negative); therefore, in this example, thetransistors 26-28 are never rendered conductive at times that wouldproduce wrong-direction torque. As can be seen, if one considers onlythe positive half-cycles of torque (each having a duration of 180°-el.),the torque has a relatively uniform level only during about 120°-el. ofthe 180°-el. half-cycle; for the approximately 30°-el. at the start andend of each half-cycle, the potential torque contribution is far frombeing of a steady value. Thus, as shown by the shaded areas, only the120°-el. intervals are actually employed; i.e., as shown by thetransistor conduction times in FIG. 4D, the respective ones of the threephases are energized by current only at the times when their torquecontributions will be of a steady value.

It is emphasized that the manner of operation show in FIG. 4 is butexemplary, it being the case that the motor produces only threeconstituent torque pulses per 360°-el. of rotor rotation, with eachtorque pulse lasting only 120°-el., not a full 180°-el. Persons skilledin the art will understand that the motor could furnish six such pulsesif, for each phase, during the 120°-el. time interval which is shiftedby 180°-el. from the respective shaded area, the coil system were to beenergized by current of reversed direction, e.g. supplied to the threephases by three further transistors or by other such means.

The substantial closing up of the stator's surface facing the air gap 16by means of the auxiliary poles 19 is highly desirable, because a largepart of the rotor magnetic field crossing over in the illustratedconstruction to the auxiliary poles would, upon omission of theauxiliary poles, be pulled to the main poles 17 and be added thereto.Functionally this would have the same effect as if the pole faces 18 ofthe main poles 17 were substantially wider than 180°-el., which would beequivalent to chording. Furthermore, strong jolting would occur. Boththese effects are avoided by means of the auxiliary poles 19. On theother hand, however, the auxiliary poles 19 hinder the installation ofthe non-overlapping coils 22, 23, 24 into the respective stator slots34.

In order on the one hand to keep small the slot openings between themain pole faces 28, but on the other hand to provide for windability ina manner suited to fabrication needs, the auxiliary poles in thethree-phase brushless dc motor of FIG. 2 (where for simplicity the rotoris not shown) are not stamped out together with the main poles from thesheet metal of the stator's sheet metal stack but instead are designedas separate pole bodies 36 which can be afterwards inserted intocorresponding recesses 37 in the stator's sheet metal stack. In thisembodiment the main poles 17 are wound with the stator coils 22, 22',23, 23' and 24, 24', during which the pole bodies 36 forming theauxiliary poles 19 are not yet inserted. Only after the winding of themain poles are the pole bodies 36 pushed into the recesses 37 in orderto substantially close up the slot openings 39. The pole bodies 36preferably consist of non-laminated, solid material. The pole bodies 36preferably consist of non-laminated, solid material. The circumferentialextent of 60°-el. of the auxiliary poles is relatively small compared tothe circumferential extent of 180°-el. of the main pole faces 18 and, incorrespondence thereto, they accept only a relatively small magnitudeflux; as a result, pole bodies 36 made from solid material do not leadto substantial eddy-current losses. The pole bodies 36 canadvantageously be fabricated from sintered material, especially sinterediron. The sintering process permits the manufacture of dimensionallyaccurate shapes without subsequent machining. Furthermore suitable forthe pole bodies 36 are siliconized iron types, such as for examplecommercially available from the Vakuumschmelze Company under the tradename "Trafoperm".

The pole bodies 36 provided to form the unwound auxiliary poles 19 canadvantageously be provided with recesses 40 (FIG. 3) to accommodate theposition sensors 30, 31, 32. Especially when the pole bodies 36 arefabricated using a sintering technique, this necessitates practically nofurther fabrication cost.

It is to be understood that, with the arrangement of FIG. 2, the rotorcan be designed in the same way as in the case of FIG. 1. Whereas FIGS.1 and 2 depict internal-rotor motors, it is furthermore to be understoodthat the expedients described above can with advantage be applied in thesame way in the case of external-rotor motors.

Concrete embodiments having been described above, the invention itselfis defined in the following claims.

What is claimed is:
 1. A three-phase collectorless dc motor comprising:apermanent-magnet rotor magnet arrangement having at least two polepairs; and a star-connected three-conductor stator winding, thewinding's conductors being arranged non-overlapping in slots of aslotted stator, wherein currents in the stator windings are controlledvia at least three semiconductor elements in response to at least threeoutput signals from a rotor position detecting circuit, said circuitincluding a plurality of magnetic-field-sensitive position sensors whichare controlled by the rotor magnet arrangement, wherein the positionsensors are distributed along the stator's circumferential direction insuch a manner relative to the stator winding's conductors that, at eachcommutation operation, each position sensor which effects thecommutation of the winding's current from one to another of thewinding's conductors is provided in a region of the stator in which acurrent-carrying coil is present neither immediately before norimmediately after the commutation operation; wherein each of theposition sensors is located substantially on a radial-symmetry axis of astator pole carrying a coil of the one phase that is not involved in thecommutation operation triggered by that particular position sensor.
 2. Athree-phase collectorless dc motor according to claim 1, wherein thenumber of stator poles stands in the ratio 3:4 to the number of rotorpoles and each of the stator poles has a breadth of substantially 180degrees-el.
 3. A three-phase collectorless dc motor according to claim1, wherein, considered in the circumferential direction, each positionsensor is located substantially midway between neighboring coils betweenwhich the commutation of the winding's current occurs under theinfluence of the position sensor in question.
 4. A three-phasecollectorless dc motor according to claim 1, wherein the motor producesat least three pulses per 360 degrees electric.
 5. A three-phasecollectorless dc motor according to claim 4, wherein the motor is asix-pole motor with at least four magnetic rotor pole pairs.
 6. Athree-phase collectorless dc motor according to claim 1, wherein at theair gap the space remaining in the circumferential direction betweeneach two neighboring stator poles is substantially filled up by anunwound auxiliary stator pole.
 7. A three-phase collectorless dc motorcomprising:a permanent-magnet rotor magnet arrangement having at leasttwo pole pairs; and a star-connected three-conductor stator winding, thewinding's conductors being arranged non-overlapping in slots of aslotted stator, wherein currents in the stator windings are controlledvia at least three semiconductor elements in response to at least threeoutput signals from a rotor position detecting circuit, said circuitincluding a plurality of magnetic-field-sensitive position sensors whichare controlled by the rotor magnet arrangement, wherein the positionsensors are distributed along the stator's circumferential direction insuch a manner relative to the stator winding's conductors that, at eachcommutation operation, each position sensor which effects thecommutation of the winding's current from one to another of thewinding's conductors is provided in a region of the stator in which acurrent-carrying coil is present neither immediately before norimmediately after the commutation operation; wherein the stator has asuccession of poles which are integrally connected to one another,wherein intermediate each pole and the next there is an auxiliary pole;wherein the auxiliary poles are provided with recesses for accommodatingthe position sensors.
 8. A three-phase collectorless dc motor accordingto claim 7, wherein the auxiliary poles are unwound poles.
 9. Athree-phase collectorless dc motor according to claim 7, wherein theauxiliary poles are inserted into recesses of the stator.
 10. Athree-phase collectorless dc motor according to claim 7, wherein eachauxiliary pole is in the form of a one-piece pole body.
 11. Athree-phase collectorless dc motor according to claim 10, wherein theauxiliary poles are fabricated from solid material.
 12. A three-phasecollectorless dc motor according to claim 11, wherein the auxiliarypoles are sintered bodies.